Such an analyte can be, for example, in the waste water field, the total organic carbon, TOC (total organic carbon content), or the total bound nitrogen TNb (Total Nitrogen, total nitrogen content) in a water sample.
In the case of known methods for determining these parameters, a liquid sample of volume of, for example, some 100 s of μl is fed to a reactor in a high temperature decomposition system. In the reactor, which, for example, is provided by a high temperature reactor formed as a pyrolysis tube, the organic ingredients are thermally decomposed to CO2 and the nitrogen containing ingredients to nitrogen oxide NOx. The acronym NOx stands here for a mixture of nitrogen oxides with nitrogen in different degrees of oxidation, which, however, has NO as the main component. In the decomposition in the high temperature reactor, there arises a gas mixture, which besides CO2 and NOx contains gaseous H2O and, in given cases, other pyrolysis- and reaction products of substances contained in the sample. The gas mixture is, with the assistance of a carrier gas (which, as a rule, also delivers the oxygen needed for the reaction) flowing permanently through the reactor, transported through a cooler having a water separator, a gas filter and an analytical unit. The amount of the occurring CO2, or NOx, is determined, for example, by infrared measurement or by chemiluminescent measurement, and, therefrom, the TOC-, or TNb, content of the liquid sample determined.
The temperatures reigning in the reactor of the high temperature decomposition system lie during operation of the analytical apparatus significantly above the boiling point of the dosed sample liquid. In the case of TOC- or TN)b determination, there rules in the interior of the reactor usually a temperature of about 650° C. up to 1300° C., depending on whether the decomposition of the sample is supported supplementally by a catalyst. In contact with the wall of the reactor or other surfaces present within the reactor, a liquid drop reaches, within a very short time, boiling temperature and, respectively, the reaction temperature required for the reaction with the oxygen contained in the carrier gas. A volume unit of the sample liquid, which e.g. can comprise one or more liquid drops dosed into the reactor transforms into the gas phase, consequently, directly after the dosing, by evaporation and/or by forming gaseous reaction products.
The described method can, on the one hand, be performed in continuous measurement operation. In such case, the sample liquid is metered in an ongoing manner with slow feed velocity, especially dropwise, into the reactor. The concentrations of the oxidation products of the analyte, e.g. the CO2—, and NOx, concentrations, respectively, in the carrier gas stream leaving the reactor are, to a first approximation, proportional to the concentration of the analyte in the sample liquid.
The method can, on the other hand, be performed in a batch fashion, in the case of which a volume unit, typically 100 to 1500 μl, of the sample liquid is decomposed in the reactor. The amount of oxidation product of the analyte contained in the carrier gas stream emerging from the reactor is correspondingly dependent both on the volume of the dosed sample liquid as well as also on the concentration of the analyte in the sample liquid.
Thus, it is clear that metering errors in the case of both methods enter proportionally the analysis result. The dosing of the sample liquid into an analytical apparatus of the initially described type occurs through one or more pumps comprising a supply line for supplying the sample liquid from a source into the reactor. Frequently in analyzers of the initially described field of the invention, one or more peristaltic pumps are used. In principle, also syringe pumps are applicable.
Syringe pumps work quite reproducibly and precisely. Their operation is, however, relatively expensive. Syringe pumps are furthermore not applicable in all fields of application for automatic analytical apparatuses. Especially, in the field of waste water analysis, due to deposition of particles present in the sample liquid or the crystallizing of solids, especially when such involve abrasive particle, the seals can be damaged, so that unsealed locations arise. Furthermore, through depositing of the particles on the inner wall of the supply line, despite pump power remaining equal, the feed rate, i.e. the volume of sample solution supplied per unit time into the reactor, can change.
A peristaltic pump is a squeeze pump, in the case of which the medium to be fed is pressed in the feed direction by external mechanical deformation, in the form of compressive stroking of a hose. Peristaltic pumps have especially the following advantages: They are inexpensive, simple to handle, the liquid comes only in contact with the hose, whereby corrosion is prevented, they are used over a large range of feed rates, namely between microliters per hour to liters per minute, and a plurality of hoses can be operated with one pump drive. Peristaltic pumps are, however, subject to the following constraints: The feed rate is not constant over the life cycle of the hose. The life cycle of a hose can be divided roughly into three phases, namely, first of all, a short break-in phase, in which the feed rate sinks moderately, second follows a long phase of relatively constant feed rate, and third, the feed rate declines toward the end of the life cycle, first slowly and then rapidly. Additionally, also in the case of a peristaltic pump, deposits on the inner wall of the hose can lead in places to cross sectional narrowing or even to clogging of the hose. Also damage to the hose, e.g. leaks, can a change the feed rate to such an extent that, eventually, the hose becomes unusable.
In order to prevent measurement errors from occurring due to a change of feed rate, different measures are used. The first position here is to be mentioned is the regular readjustment of the device using a standard solution. This readjustment requires time, however, and leads especially in the case of an analytical apparatus working in the continuous measuring mode to undesired interruptions.
Second, the supply line can be regularly cleaned, or, especially in the case of peristaltic pumps, preventitively replaced even after a relatively short time of use, e.g. when the tubes are still, with sufficiently high probability, located in the second phase of the life cycle with a stable pump behavior, even though the tubes could physically still provide service for a very long time. This means lastly an unnecessarily high maintenance effort and a waste of material.
Even in the case of regular adjustment, cleaning or in the case of early replacement of the supply line, especially the hose in the case of peristaltic pumps, it is still possible for defects, which occur due to leakages or unpredicted accreting or narrowing of the supply line, under circumstances not timely to be recognized, which leads to defective measurements.
EP 1 167 767 A1 discloses a method for monitoring an apparatus serving for producing a fluid flow for a sample collector, wherein the apparatus comprises a squeeze pump, especially a peristaltic pump, with a hose as supply line for the feed of a fluid. For monitoring the instantaneous operating state of the pump, especially the hose, in such case, an internal pressure reigning in an inlet region of the hose is measured. From pressure signals registered by means of a pressure measuring transducer arranged in an inlet region of the hose, especially the instantaneous volume flow can be ascertained. The monitoring of the pressure reigning in an inlet region of the hose gives, however, not necessarily reliable information concerning the state of the total supply line or concerning the sample volume actually metered into a vessel. In the case of an analytical apparatus of the initially described type, moreover, already low metering errors of a few drops, which corresponds to some 10 s of microliters, means, in the case of total sample volumes of 100 to 1500 μl, error in the one to two digit percent range in the analytical result. Such metering error can be caused already by a small change in the feed rate, which cannot be detected precisely enough by monitoring the liquid pressure in the supply line.